Communication by transmitting laser signals in free space (also referred to herein as “free-field”) is well known, and is referred to as free-space laser communication. It consists of modulating a laser beam, transmitted by a transmitting terminal towards a receiving terminal, according to the useful or “payload” information to be transmitted. Because of the very small beam width of the laser beam, it is necessary to keep the transmitting terminal accurately pointed in the direction of the receiving terminal during a communication session. This maintaining of the pointing direction is called the tracking phase of the communication session. Actually, both terminals must be accurately and simultaneously pointed towards each other, in particular because each of them regularly sends beacon signals which are necessary to maintain the alignment between the pointing directions, even if one of the two terminals is not sending useful information at that moment.
When a communication laser beam passes through at least part of the Earth's atmosphere, this atmosphere can interfere with the propagation of the laser beam between the two terminals, and can consequently cause transmission errors in the payload information. Difficult atmospheric conditions can even cause a loss of the optical connection between the two terminals. Document U.S. Pat. No. 6,970,651 describes a method for preventing the loss of the optical link, which is based on the use of a specific sensor for detecting an atmospheric characteristic indicative of some of the weather conditions which could affect the propagation of laser signals.
Also, document US 2006/0024061 discloses a wavefront correction of laser signals received by one of the optical communication terminals, in order to reduce the disruptions caused by atmospheric turbulence. It is known that when an optical communication is established in which one of the terminals is located in the Earth's atmosphere and the other terminal is located in space, for example on board a satellite, the optical transmission from Earth to space has more disruptions than the optical transmission in the opposite direction.